class Solution {
    string hash[10]={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    vector<string> ret;
    string path;
public:
    vector<string> letterCombinations(string digits) {
        if(digits.size()==0)return ret;
        dfs(digits,0);
        return ret;
    }
    void dfs(string& digits,int pos)
    {
        if(pos==digits.size())
        {
            ret.push_back(path);
            return;
        }

        for(auto ch:hash[digits[pos]-'0'])
        {
            path.push_back(ch);
            dfs(digits,pos+1);
            path.pop_back();
        }
    }
};
